diff --git a/lib/Transforms/Utils/CloneFunction.cpp b/lib/Transforms/Utils/CloneFunction.cpp
index b61017f37ca..314c990293c 100644
--- a/lib/Transforms/Utils/CloneFunction.cpp
+++ b/lib/Transforms/Utils/CloneFunction.cpp
@@ -325,11 +325,10 @@ void PruningFunctionCloner::CloneBlock(const BasicBlock *BB,
       // the basic block.
       if (Value *V =
               SimplifyInstruction(NewInst, BB->getModule()->getDataLayout())) {
-        assert((!isa<Instruction>(V) ||
-                cast<Instruction>(V)->getParent() == nullptr ||
-                cast<Instruction>(V)->getFunction() != OldFunc ||
-                OldFunc == NewFunc) &&
-               "Simplified Instruction should not be in the old function.");
+        // On the off-chance that this simplifies to an instruction in the old
+        // function, map it back into the new function.
+        if (Value *MappedV = VMap.lookup(V))
+          V = MappedV;
 
         if (!NewInst->mayHaveSideEffects()) {
           VMap[&*II] = V;
diff --git a/test/Transforms/Inline/pr33637.ll b/test/Transforms/Inline/pr33637.ll
new file mode 100644
index 00000000000..315feca27bd
--- /dev/null
+++ b/test/Transforms/Inline/pr33637.ll
@@ -0,0 +1,25 @@
+; RUN: opt -inline < %s
+
+define void @patatino() {
+for.cond:
+  br label %for.body
+
+for.body:
+  %tobool = icmp eq i32 5, 0
+  %sel = select i1 %tobool, i32 0, i32 2
+  br i1 undef, label %cleanup1.thread, label %cleanup1
+
+cleanup1.thread:
+  ret void
+
+cleanup1:
+  %cleanup.dest2 = phi i32 [ %sel, %for.body ]
+  %switch = icmp ult i32 %cleanup.dest2, 1
+  ret void
+}
+
+define void @main() {
+entry:
+  call void @patatino()
+  ret void
+}